1. Field of the Invention
The present invention relates to a coherent detection system and, particularly, to a coherent detection system in a mobile communication system, for synchronously detecting a wave transmitted from a high-speed mobile object through a radio transmission path.
2. Description of the Related Art
Many studies are being made on a mobile communication system using a communication satellite that covers a wide service area without many base stations. Expanding the service area of an existing ground mobile communication system has also been studied.
A ground mobile communication system involves base and mobile stations. When a mobile station moves, a radio transmission path between the mobile and base stations is disturbed by radio reflection, diffraction, scattering, etc., caused by topography and ground obstacles. In addition, many signals from other transmission paths arrive at a reception end of the radio transmission path, and therefore, the reception end is affected by considerable fading caused by the sum of the vectors of these signals.
To maintain transmission quality in spite of the fading, the phase noise, and the heat noise, the reception end usually employs a delay detector that secures a stable operation without regenerating the carrier signal from a received wave.
On the other hand, a coherent detector incorporating a carrier regenerator regenerates a carrier signal from a received wave and calculates the product of the carrier signal and received wave, to demodulate the received wave. If the scale of a fading in a radio transmission path is within an allowable range, employing a coherent detector instead of the delay detector effectively suppresses noise orthogonal to a carrier signal and improves the transmission quality.
The carrier regenerator of the coherent detector has an intrinsic loop bandwidth BL, which is usually preset to one several hundredth of a transmission speed, or several tens of times a Doppler shift, to secure a required C/N ratio, minimize a noise bandwidth, and stably regenerate a carrier according to a deviation in the frequency of the carrier of a received wave.
The speed of a low orbit satellite relative to an earth station or a mobile station on the ground is about 7 to 8 km/s to cause a Doppler shift fD of about 2 to 3 kHz in a received wave. Due to this relative speed and a large fluctuation in the relative speed, a Doppler shift in the satellite mobile communication system is quite large compared with that in the ground mobile communication system. The Doppler shift in the ground mobile communication system is only about 100 Hz when a mobile station is moving at a speed of 100 km/h.
If the Doppler shift fluctuates greatly, the loop band width BL will be out of an optimum range. Namely, when the ratio R (=BL/fD) of the loop band width BL to the Doppler shift fD greatly differs from an optimum ratio R0, the demodulation performance of a reception end deteriorates to increase a bit error rate.
The fading mentioned above causes phase noise in the coherent detector of the ground mobile communication system and stops the coherent detector regenerating a carrier signal phase-synchronized with a received wave. Then, the coherent detector is not substitutable for the delay detector.